The present invention relates to AM stereo receivers, and more particularly to a circuit for inhibiting stereo operation under certain signal conditions.
A number of different signal formatting schemes have been proposed for encoding stereo information on a carrier in the AM radio broadcasting band. In one scheme, the transmitter broadcasts what is essentially a quadrature AM (QAM) signal. A signal corresponding to the sum of the two stereo signals (i.e., L+R) is transmitted in the in-phase or I channel, and the difference between the two stereo signals (i.e., L-R) is transmitted in the quadrature or Q channel.
The QAM signal can be modified in any of a number of ways to enhance compatibility with standard monophonic receivers. For example, the gain of the Q channel signal can be maintained at a fixed level substantially below the gain of the I channel signal, or can be dynamically varied in accordance with a criterion representative of short term compatibility of the total signal with existing monophonic receivers. Alternatively, the quadrature channel can be left at full gain, but amplitude limitations applied to the L and R stereo signals so as to achieve improvements in compatibility.
Unnecessarily high levels of noise will be present in the output signals provided by a stereo receiver if it is operated in a stereo mode when receiving a conventional monophonic signal. Consequently, stereo radio receivers conventionally include circuitry for switching the receiver to a stereophonic mode of operation only when a pilot signal transmitted as part of the stereo signal is found to be present in the received signal. The same circuitry switches the receiver back to a monophonic mode of operation if the pilot signal disappears for some reason. Such circuitry thus prevents the receiver from operating in a stereophonic mode when no stereophonic information is present in the signal being received.
It would be desirable to also switch the receiver to a monophonic mode of operation when the received signal, although including stereo information, also included substantial interference. When there is significant interference, more noise free operation can be obtained in the monophonic mode. Interference in the received signal cannot be readily detected, however.